1. Field of the Invention
This invention relates to a coating composition for mold coat of a molded polyurethane resin, and more particularly to such a coating composition which has long pot life and high drying ability while having high weatherability.
2. Description of Prior Art
Hitherto so-called mold coat coating process has been proposed and put into practical use in molding polyurethane resin, in which a coating composition is coated on the inner or molding surface of a mold for polyurethane resin, prior to molding of polyurethane resin; and thereafter the coated coating composition is transferred onto the outer surface of the molded polyurethane resin during molding of the polyurethane resin. There are two types of coating compositions for the mold coat process, i.e., a single liquid type one and a two liquid type one. These types of the mold coat coating compositions will be listed hereinafter.
The single liquid type coating composition (polyurethane lacquer):
a resin constituent: polyurethane compound as addition polymer of glycols (adipate, hexane adipate and the like) and diisocyanates (no yellowing type isophorone diisocyanate, xylene diisocyanate and the like); PA1 a solvent: a mixture of cyclohexanone, methylethyl ketone, xylene, toluene, isopropanol, ethyl acetate, butyl acetate and the like; PA1 a pigment: carbon black (or organic or inorganic colouring pigment to be used for automotive vehicle body in case of other than black): PA1 a dispersant: high molecular weight polycarboxylic acid salt, polyaminoamide phosphate, or alkylamine salt of polycarboxylic acid; and PA1 a light resisting agent: a light stabilizer of benzophenone, benzotriazole or hindered amine. PA1 a principal component: PA1 solvent: a mixture of ester solvent such as ethyl acetate, butyl acetate or the like, ketone solvent such as methylisobutyl ketone, methylethyl ketone or the like, and aromatic hydrocarbon such as toluene, xylene or the like, and glycol ether such as cellosolve acetate, butyl cellosolve acetate or the like; PA1 additives: a surface improving agent of polysiloxane copolymer such as silicone oil or the like; a pigment dispersant of alkylamine salt of polycarboxylic acid or polycarboxylic acid salt; and optionally a light stabilizer of benzophenone, benzotriazole or hindered amine; PA1 pigment: inorganic pigment of carbon black, titanium dioxide or the like, organic pigment of Phthalocyanine Blue, quinacridone red or the like, extender pigment of calcium carbonate, barium sulfate or the like, and optionally inorganic substance such as silica or the like and organic substance such as polyethylene wax or the like (these substances are usually used as delustering agent); and PA1 a hardener component:
The two liquid type coating composition:
a resin component (a): polyester polyol (having a molecular weight of 800 to 3,000, hydroxyl value of 12 to 150 and an acid value of not more than 4) which is a polymer of polybasic acid (adipic acid, dicarboxylic acid such as phthalic acid) and polyol of polyhydric alcohol (polyol having OH group at its chain end, such as ethylene glycol, trimethylol propane and the like); PA2 a resin constituent (b): a mixture of polyester polyol (or the resin constituent (a)) and acryl polyol whose principal ingredient is alkyl acylate containing stylene or alkylmethacrylate; PA2 a resin constituent (c): a mixture of the above-mentioned resin component (b) and a copolymer of vinyl chloride and vinyl acetate, or of the above-mentioned resin component (b) and cellulose resin such as cellulose acetate butyrate; and/or PA2 a resin constituent (d): acryl polyol of hydroxyacryl copolymer (having a molecular weight of 3,000 to 10,000, an acid value of 2 to 9 and a hydroxy value of 5 to 30) of acrylate containing hydroxy such as hydroxyalkyl methacrylate and hydroxyalkyl acrylate, and acrylic esters (MMA, butyl methacrylate and the like); PA2 a resin constituent: aliphatic diisocyanate prepolymer (for example, no yellowing diisocyanate such as hexamethylene diisocyanate, isophorone diisocyanate, xylene diisocyanate or the like), aromatic diisocyanate such as T.D.I, hydrogenenated T.D.I, or so-called soft isocyanate of addition polymer of aliphatic diisocyanate and higher alcohol or polyester to introduce a soft segment into a structure for softening purpose; and PA2 a solvent constituent: ester solvent such as ethyl acetate, butyl acetate or the like, aromatic hydrocarbon such as toluene, xylene or the like, glycol ester such as cellosolve acetate or the like, or a mixture of them.
However, difficulties have been encountered in the above-listed single and two liquid type mold coat coating compositions, as follows: In the above single liquid type mold coat coating composition, the bond of the principal chain of the structure is an ester bond and therefore tends to be broken by hydrolysis. In the above two liquid type mold coat coating composition, the embodiment whose principal component is polyester polyol has ester bonds as the principal chain bonds and has benzene ring in the structure. The embodiment whose principal component is acryl polyol has a rigid molecular chain. Accordingly, both compositions are lower in reaction rate and therefore inferior in drying ability which is required for a coating composition for mold coat. This requires simultaneous use of reaction accelerator in order to improve drying ability, particularly to accelerate initial drying. As a result of use of the reaction accelerator, the single liquid type coating composition is inferior in moisture resistance, hot water resistance and weatherability. Similarly, the above two liquid type coating compositions respectively including, as the principal component, polyester polyol and acryl polyol have defective characteristics as shown in Table 1.
TABLE 1 ______________________________________ Conventional coating composition Principal compo- Principal nent: component: Characteristics polyester polyol acryl polyol ______________________________________ Weatherability Bad (chalking Good occurred, luster lowered) Bending ability Poor Poor or bad after weather- (crack ability test produced) Pot life Poor or bad Bad Moisture Poor Good resistance (whitened) Hot water Poor Good resistance Open time Shortened Not shortened (limited) ______________________________________